Is dark energy, a hypothetical form of energy that acts in opposite to gravity at large scales, an illusion?

Astronomers were astonished to discover in 1998 that the expansion of the universe is happening at an ever-increasing rate. The mysterious repulsive force responsible for this was dubbed dark energy, though scientists still do not know what it is.

Now, physicist Syksy Rasanen of CERN in Geneva, Switzerland, says we might not need dark energy after all. As counter-intuitive as it sounds, the increasing rate of expansion might be due to the collapse of small regions of the universe under gravity, he says.